60 R. HOLMAN PECK. 



relation to tlie vertical vessels, is accompanied by a further 

 difference of form, which is obvious when the sections given 

 in Plate V, figs. 2 and 3, are compared. 



In the outer gill-plate the two lamellae are parallel to one 

 another and of equal thickness. In the inner gill-plate the 

 outer lamella is thicker than the inner, and its surface is 

 thrown into a series of curves. It bulges out considerably 

 betAveen each fixed line of interlamellar junction, so that we 

 have a slight indication here of that plication of the gill- 

 lamella which becomes so marked and important a feature 

 of the architecture of the gill-plates in the so-called com- 

 pound gills of Ostrea, Cardium, &c. 



Structure of the Filaments. — The figures of transverse sec- 

 tions and surface views of the filaments given in the plates 

 and the accompanying explanation will serve better than a 

 long description, to place the results of my observations 

 before the reader. In the main, they agree with Posner's, 

 but I differ from him as to the epithelium, and also as to the 

 cavity which represents the original vascular channel of the 

 primitive gill-filament, as we see it in Area and Mytilus. 



The filaii ents have a uniform structure throughout the gill- 

 plates of Anodon. They have lost their simple tubular char- 

 acter by (a) the resolution and outgrowth of their deep or 

 sub-lamellar walls whereby the mass of sub-filameutar tissue 

 is formed and {b) by the excessive thickening of the chitinous 

 deposit, which whilst vastly increased on three sides of the 

 original cavity of the filament, is altogether absent on the deep 

 sides where the filament breaks out into a loose sub-filamen- 

 tar growth of lacunar tissue (Plate V, fig. 12). In fact, from 

 being hollow, tubes kept open by a firm deposit of chitinous 

 matter the filaments have become comparatively solid rods 

 with a median fissure corresponding with the primitive cavity, 

 and as rods they merely support the loose lacunar (therefore 

 blood-permeated) tissue which grows out beneath them. 



There are no ciliated interfilamentar junctions in Anodon. 

 In place of these we have very solid fibrous concrescences 

 or junctions. The fibrous interfilamentar junctions of the 

 Anodon's gill form regular transverse bands continuous hori- 

 zontally across the series of filaments, and thus divide the 

 long though narrow interfilamentar spaces into a series of 

 rectangular spaces, about three times as long in vertical as in 

 transverse measurement (PL VI, fig. 14). The surface epithe- 

 lium of the gill-filaments is, of course, continued on to the fibrous 

 interfilamentar junctions where it is continued inwards to 

 clothe the water-passages and interlamellar surface. Each of 

 the oblong rectangular spaces enclosed by the filaments and 



